Bone anchors

ABSTRACT

A cannulated bone anchor assembly includes a bone anchor, a receiver member for receiving a spinal fixation element to be coupled to the bone anchor, and a closure mechanism to fix the spinal fixation element with respect to the receiver member. The bone anchor includes a distal shaft having a first threaded section distal to a proximal head, a second threaded section distal to and adjacent the first threaded section, and a third threaded section distal to and adjacent the second threaded section. The first threaded section having a constant major and minor diameter. The second threaded section having a tapering major and minor diameter. The third threaded section having a tapering major diameter and a constant minor diameter.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/425,496, filed May 29, 2019, which is a continuation of U.S. patentapplication Ser. No. 14/855,919, filed Sep. 16, 2015, which granted asU.S. Pat. No. 10,321,937 and which is a continuation of U.S. patentapplication Ser. No. 13/591,811 filed Aug. 22, 2012, which granted asU.S. Pat. No. 9,155,580 and which claims the benefit of U.S. ProvisionalApplication No. 61/527,389, filed Aug. 25, 2011. Each of theafore-mentioned patent applications is incorporated herein by reference.

BACKGROUND

Bone anchors may be used in orthopedic surgery to fix bone during thehealing or fusion process. In spinal surgery, bone anchors may be usedwith spinal fixation elements, such as spinal rods, to stabilizemultiple vertebrae either rigidly, in which no relative motion betweenthe vertebrae is desired, and dynamically, in which limited, controlledmotion between the vertebrae is desired. In certain procedures, such asminimally invasive procedures, it may be desirable to deliver the boneanchor to the vertebra over a guide wire or other guidance instrument.To facilitate this, bone anchors may be cannulated, i.e., may have acentral passage through the bone anchor that is sized to receive theguide wire. One problem with the use of cannulated bone anchors is thatwhen such bone anchors are used in a procedure without a guide wire, forexample, in an open or non-minimally invasive procedure, it may bedifficult to start the bone anchor in the proximal surface of the boneunless either an opening in the bone is pre-threaded using a tap orother instrument or the surgeon applies a significant distal force onthe bone anchor. Accordingly, there is need for improved cannulated boneanchors that facilitate placement of the anchor in the bone.

SUMMARY

Disclosed herein are improved bone anchor assemblies and, in particular,improved bone anchor assemblies used in connection with spinal fixationelements to fix multiple vertebrae either rigidly or dynamically.

In accordance with one aspect, a bone anchor assembly may include a boneanchor, a receiver member for receiving a spinal fixation element to becoupled to the bone anchor, and a closure mechanism to fix the spinalfixation element with respect to the receiver member. The bone anchormay include a distal shaft having a first threaded section distal to aproximal head, a second threaded section distal to and adjacent thefirst threaded section, and a third threaded section distal to andadjacent the second threaded section. The first threaded section mayhave a constant major and minor diameter. The second threaded sectionmay have a tapering major and minor diameter. The third threaded sectionmay have a tapering major diameter and a constant minor diameter. Thebone anchor may include a central passage extending from the proximalhead through the distal shaft.

BRIEF DESCRIPTION OF THE FIGURES

These and other features and advantages of the devices and methodsdisclosed herein will be more fully understood by reference to thefollowing detailed description in conjunction with the attached drawingsin which like reference numerals refer to like elements through thedifferent views. The drawings illustrate principles of the devices andmethods disclosed herein and, although not to scale, show relativedimensions.

FIG. 1 is a side view of an exemplary embodiment of a bone anchorassembly;

FIG. 2 is a side view in cross section of the bone anchor assembly ofFIG. 1;

FIG. 3 is a side view of the bone anchor of the bone anchor assembly ofFIG. 1;

FIG. 4 is a side view in cross section of the bone anchor of the boneanchor assembly FIG. 1;

FIG. 5 is a side view of another exemplary embodiment of a bone anchorassembly;

FIG. 6 is a side view in cross section of the bone anchor assembly FIG.5;

FIG. 7 is an exploded view of the bone anchor assembly FIG. 5;

FIG. 8 is a side view of another exemplary embodiment of a bone anchorassembly;

FIG. 9 is a side view of the bone anchor assembly FIG. 8, illustratingthe bone anchor rotated 90° from FIGS. 8; and

FIG. 10 is a side view in cross section of the bone anchor assembly FIG.8.

DETAIL DESCRIPTION OF EXEMPLARY EMBODIMENTS

Certain exemplary embodiments will now be described to provide anoverall understanding of the principles of the structure, function,manufacture, and use of the devices and methods disclosed herein. One ormore examples of these embodiments are illustrated in the accompanyingdrawings. Those of ordinary skill in the art will understand that thedevices and methods specifically described herein and illustrated in theaccompanying drawings are non-limiting exemplary embodiments and thatthe scope of the present invention is defined solely by the claims. Thefeatures illustrated or described in connection with one exemplaryembodiment may be combined with the features of other embodiments. Suchmodifications and variations are intended to be included within thescope of the present invention.

The articles “a” and “an” are used herein to refer to one or to morethan one (i.e. to at least one) of the grammatical object of thearticle. By way of example, “an element” means one element or more thanone element.

The terms “comprise,” “include,” and “have,” and the derivativesthereof, are used herein interchangeably as comprehensive, open-endedterms. For example, use of “comprising,” “including,” or “having” meansthat whatever element is comprised, had, or included, is not the onlyelement encompassed by the subject of the clause that contains the verb.

FIGS. 1-4 illustrate an exemplary embodiment of a bone anchor assembly10 including a bone anchor 12, a receiver member 14 for receiving aspinal fixation element, such as a spinal rod, to be coupled to the boneanchor 12, and a closure mechanism 16 to capture a spinal fixationelement within the receiver member 14 and fix the spinal fixationelement with respect to the receiver member 14. The bone anchor 12includes a proximal head 18 and a distal shaft 20 configured to engagebone. The receiver member 14 has a proximal end 26 having a pair ofspaced apart arms 28A, 28B defining a recess 30 therebetween and adistal end 32 having a distal end surface 34 defining opening throughwhich at least a portion of the bone anchor 12 extends. The closuremechanism 16 may be positionable between and may engage the arms 28A,28B to capture a spinal fixation element within the receiver member 14and fix the spinal fixation element with respect to the receiver member14.

Continuing to refer to FIGS. 1-4, the proximal head 18 of the boneanchor 12 in the exemplary embodiment is generally in the shape of atruncated sphere having a planar proximal surface 36 and anapproximately spherically shaped distal surface 38. The exemplary boneanchor assembly is a polyaxial bone screw designed for posteriorimplantation in the pedicle or lateral mass of a vertebra. In thisregards, the proximal head 18 of the bone anchor 12 engages the distalend 32 of the receiver member 14 in a ball and socket like arrangementin which the proximal head 18, and thus the distal shaft 20, can pivotrelative to the receiver member 14. The distal surface 38 of theproximal head 18 of the bone anchor 12 and the mating surface within thedistal end 32 of the receiver member 14 may have any shape thatfacilitates this ball and socket like arrangement, including, forexample, spherical (as illustrated), toroidal, conical, frustoconical,and any combinations of these shapes.

The distal shaft 20 of the bone anchor 12 may be cannulated, having acentral passage or cannula 40 extending the length of the bone anchor 12to facilitate delivery of the bone anchor 12 over a guide wire in, forexample, minimally invasive procedures. The other components of the boneanchor assembly, including the closure member 16, the receiver member14, and the compression member 90 (discussed below) may be cannulated orotherwise have an opening to permit the respective component to bedelivered over a guide wire.

Continuing to refer to FIGS. 1-4, the proximal end 26 of the receivermember 14 of the exemplary bone anchor assembly 10 includes a pair ofspaced apart arms 28A, 28B defining the U-shaped recess 30 therebetweenfor receiving a spinal fixation element. The distal end 32 of thereceiver member 14 is generally cylindrical in shape and includes distalend surface 34 which is generally annular in shape defining a circularopening through which at least a portion of the bone anchor 12 extends.For example, the distal shaft 20 of the bone anchor 12 may extendthrough the opening. Each arm 28A, 28B of the proximal end 26 of thereceiver member 14 extends from the distal end 32 of the receiver member14 to a free end. The outer surface of each arm 28A, 28B may include afeature, such as a recess, dimple, notch, projection, or the like, tofacilitate connection of the receiver member 14 and, thus, the boneanchor assembly 10, to instruments. In the exemplary embodiment, forexample, the outer surface of each arm 28A, 28B includes an arcuategroove 44A, 44B at the respective free end of the arms. Such grooves aredescribed in more detail in U.S. Pat. No. 7,179,261, which isincorporated herein by reference.

The proximal end 26 of the receiving member 14 may be configured toreceive a closure mechanism, such as internal set screw (closuremechanism 16) or an external cap or nut. For example, the interiorsurface of each arm 28A, 28B may include a feature, such as a recess,dimple, notch, projection, thread or the like, to facilitate connectionof the closure mechanism 16 to the receiver member 14. In the exemplaryembodiment, for example, the interior surface of each arm 28A, 28Bincludes an internal thread 46 on the interior surface of each arm 28A,28B for engaging the closure mechanism 16. In the exemplary embodiment,the thread starts at the free, proximal end and extends distally alongat least a portion of the length of the arms 28A, 28B.

The closure mechanism 16 in the exemplary embodiment is an internal setscrew having an external thread that engages the internal thread of thereceiver member to capture a spinal fixation element within the recess30 of the receiver member and, when fully tightened, to fix the spinalfixation element relative to the receiver member 14. Alternatively, theclosure mechanism may be dual closure mechanism having an inner and anouter set screw, such as, for example, the Expedium Dual Innie PolyaxialScrew available from DePuy Spine, Inc. of Raynham, Mass. In addition,the closure mechanism may be a non-threaded twist-in cap, such as, forexample, the Monarch Typhoon Cap available from DePuy Spine, Inc. ofRaynham, Mass., and described in U.S. Pat. No. 6,755,829, incorporatedherein by reference.

The exemplary bone anchor assembly 10 may be used with a spinal fixationelement such as a rigid spinal rod. The spinal rod may be constructedtitanium, titanium alloys, stainless steel, cobalt chrome, PEEK, orother materials suitable for rigid fixation. Alternatively, the spinalfixation element may be a dynamic stabilization member that allowscontrolled mobility between the instrumented vertebrae.

The exemplary bone anchor assembly is a rigid polyaxial screw in whichthe bone anchor 12 is fixed, rather than mobile, when the spinalfixation element is fixed to the receiver member 14 of the bone anchorassembly. The spinal fixation element may either directly contact theproximal head 18 of the bone anchor 12 or may contact an intermediateelement, e.g., a compression member 100, interposed between the spinalfixation element and the proximal head 18 of the bone anchor 12 tocompress the distal outer surface of the proximal head 18 into direct,fixed engagement with the distal inner surface of the receiver member 18when the spinal fixation element is fixed to the receiver member 16 ofthe bone anchor assembly by the closure mechanism. In alternativeembodiments, the bone anchor assembly may be a mobile screw in which theproximal head 18 of the bone anchor 12 can move relative to the receivermember 14 when the spinal fixation element is fixed to the receivermember 14. An exemplary mobile polyaxial screw is described is U.S.Patent Application Publication No. US 2011-0093021, which is herebyincorporated herein by reference. Alternatively, the bone anchorassembly may be a monoaxial screw, a favored angle screw or a uniplanarscrew.

Referring to FIGS. 3 and 4, the distal shaft 20 may be threaded toimprove fixation of the bone anchor 12 in bone, such as the pedicle orlateral mass of a vertebra, and, in particular, to facilitate startingthe bone anchor 20 in the proximal surface of the bone. In the exemplaryembodiment, the distal shaft 20 has a first threaded section 50 distalto the proximal head 18, a second threaded section 52 distal to andadjacent the first threaded section 50, and a third threaded section 54distal to and adjacent the second threaded section 52. The firstthreaded section has a first major diameter 56, a first minor diameter58, and a first length 60. The first major diameter 56 and the firstminor diameter 58, in the exemplary embodiment, are constant over thecomplete first length 60 of the first threaded section 50. The secondthreaded section 52 has a second major diameter 62, a second minordiameter 64, and a second length 66. In the exemplary embodiment, thesecond major diameter 62 and the second minor diameter 64 taper in aproximal to distal direction over the complete second length 66 of thesecond threaded section 52. The second major diameter 62 and the secondminor diameter 64 may be less than the first major diameter 56 and thefirst minor diameter 58 over the complete second length 66 of the secondthreaded section 52. The third threaded section 54 has a third majordiameter 68, a third minor diameter 70, and a third length 72. In theexemplary embodiment, the third major diameter 68 tapers in a proximalto distal direction over the complete third length 72 of the thirdthreaded section 54 and the third major diameter 68 is less than thesecond major diameter 62 over the complete third length 72 of the thirdthreaded section 54. The third minor diameter 70 may be constant overthe complete third length 72 of the third section 54 and the third minordiameter may be equal to the second minor diameter 70 at a transitionfrom the second threaded section 52 to the third threaded section 54.

In the exemplary embodiment, the distal shaft 20 includes an unthreadeddistal tip 80 distal to and adjacent to the third threaded section 54.The unthreaded tip 80 may taper in a proximal to distal direction andmay be generally conical in shape. In the exemplary embodiment, theangle of the cone 82 formed by the unthreaded tip 80 is approximately70°. The unthreaded tip is provided to minimize sharp edges,particularly from the thread, at the tip and thereby minimize softtissue damage during delivery of the bone anchor to the bone surface.The axial length of the unthreaded distal tip 80 is preferably minimizedin order to provide the thread as far distally on the shaft as possible.

The tapering minor diameter 64 in the second threaded section 52 and thetapering major diameters 62 and 68 in the second threaded section 52 andthird threaded section 54, respectively, facilitate starting of the boneanchor in bone. Moreover, the exemplary thread form eliminates the needfor a cutting flute on the distal end on the distal shaft 20. To preventcollapse of the cannula 40 during insertion, a minimum wall thickness inthe third section 54 and unthreaded section 80 is preferably maintained.The minimum wall thickness, i.e., the difference between the minordiameter 70 and the diameter of the cannula 40, is preferably greaterthan 0.5 mm. The minor diameter 64 of the second section 52 preferablytapers until the desired minimum wall thickness is obtained. At thispoint along the axial length of the distal shaft 20, the second threadedsection 52 transitions to the third threaded section 54 and the minordiameter 70 remains constant.

FIGS. 5-7 illustrate another exemplary embodiment of a bone anchorassembly 100 in which the bone anchor 112 is a two-piece constructionpermitting the distal shaft 120 of the bone anchor 112 to be assembledthrough the opening in the distal surface 34 of the receiver member 14.In the exemplary embodiment, the bone anchor 112 includes a threadeddistal shaft 120 having an analogous thread form to the thread form ofthe distal shaft 20 of the bone anchor 12 described above. The proximalhead of the bone anchor 112 includes two separable components: acylindrically shaped, non-threaded proximal section 102 at the proximalend of the distal shaft 120 and a resiliently compressible,spherically-shaped retaining member 104 that accepts and retains theproximal section 102 within the retaining member 104. The proximalsection 102 includes an annular projection or rib 106 configured to seatwithin a complementary shaped annular groove 108 provided on the innersurface of the retaining member 104. During assembly, retaining member104 is positioned within the receiver member 14. The retaining member104 can be compressed to facilitate positioning in the receiver member14 after which the retaining member 104 returns to its nominalconfiguration. The proximal section 102 is inserted through the openingin the distal end surface 34 and into the retaining member 104. Theannular projection 106 is seated within the annular groove 108 to retainthe proximal section 102 within the retaining member 104, and, thus, thereceiver member 14.

FIGS. 8-10 illustrate another exemplary embodiment of a bone anchorassembly 200, in which the bone anchor 212 is configured to facilitateengagement of the bone anchor in bone. In the exemplary embodiment, thefirst threaded section 250 of the distal shaft 220 of the bone anchor212 includes distal threaded section 204 and a proximal threaded section202. The distal threaded section 204, the proximal threaded section 202,and the second threaded section 52, and third threaded section 54 of thedistal shaft 220 of the bone anchor 212 may be configured to increasefixation of the bone anchor assembly 10 in bone. The distal threadedsection 204 may have a first pitch and a first number of thread startsand the proximal threaded section 202 may have a second pitch less thanthe first pitch and a second number of thread starts greater than thefirst number of thread starts. The distal threaded section 204 and theproximal threaded section 202 may have a constant lead.

For a bone anchor assembly designed to be implanted through the pedicleof a vertebra, for example, the threaded distal section 204, and thesecond threaded section 52 and the third threaded section 54, may beconfigured to engage the cancellous bone in the anterior vertebral bodyof the vertebra and the threaded proximal section 202 may be configuredto engage the cortical bone of the pedicle of the vertebra. Inparticular, the threaded distal section 204, and the second threadedsection 52 and third threaded section 54, may have a pitch that isgreater than (i.e., more coarse) the pitch of the proximal section 202.In the exemplary embodiment, the threaded distal section 204, the secondthreaded section 52 and third threaded section 54 have the same pitch.

To facilitate insertion of the bone anchor 212 into the vertebra andprevent stripping of the pedicle wall, the distal shaft 220, includingthe threaded proximal section 202, threaded distal section 204, thesecond threaded section 52 and third threaded section 54, can have aconstant thread lead. The lead of a thread is the distance the distalshaft 220 travels in a direction parallel to the longitudinal axis ofthe shaft when the distal shaft 220 is rotated one turn (360°). The leadof a thread is equal to the number of thread starts multiplied by thepitch of the thread. As the threaded distal section 204, the secondthreaded section 52, and the third threaded section 54 have a differentpitch than the threaded proximal section 202, the threaded distalsection 204, the second threaded section 52, and the third threadedsection 54 must have a different number of thread starts than thethreaded proximal section 202 in order to have a constant or equal lead.In the exemplary polyaxial bone anchor assembly 200, for example, thelead of the distal shaft 220 is 6 mm, the pitch of distal threadedsection 204, the second threaded section 52, and the third threadedsection 54 is 3 mm and the distal threaded section 204, the secondthreaded section 52, and the third threaded section 54 have two threadstarts (i.e., the distal threaded section 204, the second threadedsection 52, and the third threaded section 54 is dual threaded) and thepitch of proximal threaded section 24 is 1.5 mm and the proximalthreaded section 202 has four thread starts (i.e., the proximal threadedsection 24 is quad threaded). Further exemplary bone anchors having athread form for increased bone fixation are disclosed in U.S. patentapplication Ser. No. 13/110378, filed May 18, 2011, which isincorporated herein by reference.

The lead of the threaded distal section 204, the second threaded section52, the third threaded section 54, and the threaded proximal section 24can vary depending on, for example, the type of bone anchor assembly(e.g., polyaxial, monoaxial, uniplanar) and the vertebra or other bonein which the assembly is to be implanted. For polyaxial bone anchorsdesigned to be inserted through the pedicle of a lumbar or thoracicvertebra, for example, the lead may be from 4 mm to 8 mm and the pitchof the distal threaded section 204, the second threaded section 52, andthe third threaded section 54, may be from 2 mm to 4 mm, and the pitchof the proximal threaded section 202 may be from 1 mm to 3 mm. Inmonoaxial screws, for example, the lead may be 2 mm to 4 mm.

The axial length (i.e., the length in a direction parallel to thelongitudinal axis of the distal shaft 220) of the proximal threadedsection 202 of the distal shaft 220 can vary depending on the vertebraor other bone in which the assembly is to be implanted and may beselected to correspond to the length of bone the proximal threadedsection 202 will engage. For bone anchors designed to be insertedthrough the pedicle of a lumbar or thoracic vertebra, the axial lengthof the proximal threaded section 202 may be selected to approximate thelength of the pedicle including the distance from the posterior surfaceof the vertebra through the pedicle to the junction of the pedicle andthe anterior vertebral body of the vertebra. In such bone anchors, theaxial length of the proximal threaded section 202 may be between 14 mmand 26 mm and preferably is 20 mm. The axial length of the distal shaft220 may also vary depending on the bone in which the bone anchor 212 isto be inserted. For bone anchors designed to be inserted through thepedicle of a lumbar or thoracic vertebra, the axial length 260 of thedistal shaft 220 may be between 20 mm and 100 mm. For bone anchorsdesigned to be inserted through the iliac, the axial length 260 of thedistal shaft 220 may be between 60 mm and 150 mm.

The major diameter and the minor diameter of the proximal threadedsection 202 and the distal threaded section 204 may be selected based onthe bone in which the bone anchor 212 is to be inserted. For boneanchors designed to be inserted through the pedicle of a lumbar orthoracic vertebra (such as the exemplary bone anchor 212), for example,the major diameter of the distal threaded section 204 and the proximalthreaded section 202 may be between 4 mm and 10 mm. In the exemplaryembodiment, the major diameter of the distal threaded section 204 andthe major diameter of the proximal threaded section 202 are equal andconstant over the axial length of the distal threaded section 204 andthe proximal threaded section 202. In certain exemplary embodiments, theminor diameter of the distal threaded section 204 and the minor diameterof the proximal threaded section 202 are equal and constant over theaxial length of the distal threaded section 204 and the proximalthreaded section 202. In other exemplary embodiments, the minor diameterof the proximal threaded section 202 is greater than the minor diameterof the distal threaded section 204. The increased minor diameter of theproximal threaded section 202 provides reduced thread depth for theproximal threaded section 202 which increases bone purchase bycompressing the bone of the pedicle of the vertebra.

The distal shaft 220 may also include one or more side wall openings 211or fenestrations that communicate with the cannula 40 to permit bonein-growth or to permit the dispensing of bone cement or other materialsthrough the bone anchor assembly 200. The side wall openings 211 extendradially from the cannula 40 through the side wall of the distal shaft220. Exemplary systems for delivering bone cement to the bone anchorassembly 200 and alternative bone anchor configurations for facilitatingcement delivery are described in U.S. Patent Application Publication No.2010/0114174, which is hereby incorporated herein by reference. Thedistal shaft 220 of the bone anchor 212 may also be coated withmaterials to permit bone growth, such as, for example, hydroxyl apatite,and the bone anchor assembly 200 may be coated all or in-part withanti-infective materials, such as, for example, tryclosan.

While the devices and methods of the present invention have beenparticularly shown and described with reference to the exemplaryembodiments thereof, those of ordinary skill in the art will understandthat various changes may be made in the form and details herein withoutdeparting from the spirit and scope of the present invention. Those ofordinary skill in the art will recognize or be able to ascertain manyequivalents to the exemplary embodiments described specifically hereinby using no more than routine experimentation. Such equivalents areintended to be encompassed by the scope of the present invention and theappended claims.

1-8. (canceled)
 9. A bone anchor, comprising: a proximal head; and adistal shaft extending distally from the proximal head and configured toengage bone, the distal shaft having a first threaded section distal tothe proximal head, a second threaded section distal to and adjacent thefirst threaded section, and a third threaded section distal to andadjacent the second threaded section, the first threaded section havinga first major diameter, a first minor diameter, and a first length, thefirst major diameter and the first minor diameter being constant overthe first length of the first threaded section, the first threadedsection having a proximal portion with a first number of thread startsand a distal portion with a second number of thread starts, the secondnumber of thread starts being less than the first number of threadstarts, the second threaded section having a second major diameter and asecond length, the second major diameter tapering in a proximal todistal direction over the second length of the second threaded section,the second threaded section having the second number of thread starts,and the third threaded section having a third major diameter and a thirdlength, the third major diameter tapering in a proximal to distaldirection over the third length of the third threaded section, the thirdthreaded section having the second number of thread starts.
 10. The boneanchor of claim 9, wherein the first number of thread starts is four andthe second number of thread starts is two.
 11. The bone anchor of claim9, wherein the distal shaft is cannulated.
 12. The bone anchor of claim9, wherein the distal shaft includes a central passage and a pluralityof side wall openings that communicate with the central passage.
 13. Thebone anchor of claim 12, wherein the side wall openings extend radiallyfrom the central passage through a side wall of the distal shaft. 14.The bone anchor of claim 9, wherein the head comprises a retainingmember separate from and assembled onto the distal shaft.
 15. The boneanchor of claim 9, wherein the head is integrally formed with the shaft.16. The bone anchor of claim 9, further comprising a receiver memberhaving a pair of spaced apart arms configured to receive a spinal rodtherebetween, the receiver member being pivotally coupled to the boneanchor.
 17. The bone anchor of claim 9, wherein the second threadedsection has a second minor diameter that tapers in a proximal to distaldirection over the second length of the second threaded section.
 18. Thebone anchor of claim 9, wherein the third threaded section has a thirdminor diameter that is constant over the third length of the thirdthreaded section.
 19. The bone anchor of claim 9, wherein the shaftincludes an unthreaded distal tip distal to and adjacent to the thirdthreaded section, the unthreaded tip tapering in a proximal to distaldirection and being generally conical in shape.
 20. A bone anchor,comprising: a proximal head; and a distal shaft extending distally fromthe proximal head and configured to engage bone, the distal shaft havinga first threaded section distal to the proximal head, a second threadedsection distal to and adjacent the first threaded section, and a thirdthreaded section distal to and adjacent the second threaded section, thefirst threaded section having a first major diameter, a first minordiameter, and a first length, the first major diameter and the firstminor diameter being constant over the first length of the firstthreaded section, the first threaded section having a first number ofthread starts, the second threaded section having a second majordiameter and a second length, the second major diameter tapering in aproximal to distal direction over the second length of the secondthreaded section, the second threaded section having a second number ofthread starts, and the third threaded section having a third majordiameter and a third length, the third major diameter tapering in aproximal to distal direction over the third length of the third threadedsection, the third threaded section having a third number of threadstarts, wherein the first, second, and third number of thread starts arethe same.
 21. The bone anchor of claim 20, wherein the first, second,and third number of thread starts are each two.
 22. The bone anchor ofclaim 20, wherein the distal shaft is cannulated.
 23. The bone anchor ofclaim 20, wherein the distal shaft includes a central passage and aplurality of side wall openings that communicate with the centralpassage.
 24. The bone anchor of claim 23, wherein the side wall openingsextend radially from the central passage through a side wall of thedistal shaft.
 25. The bone anchor of claim 20, wherein the headcomprises a retaining member separate from and assembled onto the distalshaft.
 26. The bone anchor of claim 20, wherein the head is integrallyformed with the shaft.
 27. The bone anchor of claim 20, furthercomprising a receiver member having a pair of spaced apart armsconfigured to receive a spinal rod therebetween, the receiver memberbeing pivotally coupled to the bone anchor.
 28. The bone anchor of claim20, wherein the second threaded section has a second minor diameter thattapers in a proximal to distal direction over the second length of thesecond threaded section.
 29. The bone anchor of claim 20, wherein thethird threaded section has a third minor diameter that is constant overthe third length of the third threaded section.
 30. The bone anchor ofclaim 20, wherein the shaft includes an unthreaded distal tip distal toand adjacent to the third threaded section, the unthreaded tip taperingin a proximal to distal direction and being generally conical in shape.